DH220 Dental Materials

Lecture #9

Prof. Lamanna RDH, MS

GYPSUM:Plaster and Stone

Bird & Robinson p. 804 – 806, 812 - 814

I. Use – to produce a positive reproduction of

the oral structures

A. Plaster1. 1º purpose is to fabricate study models for tx planning

- diagnostic aid – view mouth from all angles

- serves as a permanent record of the pt’s

occlusion & alignment of their teeth prior to tx

- visual aid during case presentation & pt ed

2. Mounting casts to articulator - reproduces

the “hinge” motion of the jaw

B. Stone

- produces working casts (replica of quad or

arch) & to form dies (replica of a prepared

tooth) to fabricate cast restoratives (inlays,

onlays, crowns, bridges, RPDs, & full

dentures)

cast

die

cast & die

II. Composition & Types

• Class of material – ceramic due to physical properties

• Chemically identical; difference is physical form (shape of powder

particles) of the compound calcium sulfate dihydrate.

• Calcination process:

– Plaster – gypsum in rock form, ground & heated

to remove water

- Powder particles – porous & rough

– beta-hemihydrate.

– Stone - gypsum in rock form, ground & heated

under steam pressure to remove water.

- Powder particles – smooth & dense

– alpha-hemihydrate.

• Modifiers – added to regulate setting time & expansion

– Potassium sulfate – accelerator

– Borax – retarder & reduce setting expansion

• Pigments – buffs & pastels added to distinguish between

plaster (white) & stone (buff & pastels)

Composition….con’t

Types – ADA classification system

• 2 plasters & 3 stones - variance in properties & handling

• Strength increases from Type I V by lowering W/P ratio

• 5 Types –

– TYPE I: Impression plaster – rigid, edentulous pts only

– TYPE II: Model plaster – construct study models (Tx plan)

– TYPE III: Dental stone – casts for denture fabrication (RPD, RFD)plus:

– TYPE IV: Die stone – casts for in/onlays, crowns, bridge fabrication» High strength, hardness & low expansion

– TYPE V: High strength, high expansion stone – newest edition; accommodates the greater shrinkage of newer alloys

III. Strength & Hardness

• Directly related to amount of water added to powder.

• More water = weaker mix

• Plaster = more water = weaker material

• Stone = less water = stronger material

• As material dries, entrapped water evap, leaves voids = porous, weak model/cast

• Inadequate strength results in fracture or distortion

• Reaches maximum strength after 24 hrs

How much water??

IV. Handling & Mixing

• Setting reaction – chemical - results in exothermic

reaction What is that??

A. Water – controls strength, rigidity, & hardness

1. Formation of gypsum “crystals” – cluster formation;

center of each cluster: nuclei of crystallization –

intermeshing of crystals gives final strength & rigidity

2. Water to Powder (W/P) ratio

- too much water added – increase in setting time, fewer crystals – reduced strength

- not enough water – reduces setting time, less working time, crumbly mixture of reduced strength

3. Water temperature (68º - 72º F)- ↑ temp accelerates set

- ↓ temp prolongs set

Crystals unable to

accurately intermesh.

B. Armamentarium

1. Powder (gypsum) & water; measuring vial

* plaster = ___ ml / 100 g (W/P)

* dental stone = ___ ml / 100 g (W/P)

* die stone = ___ ml / 100 g (W/P)

2. rubber bowl

3. spatula – wide & narrow blade

4. scale – measure powder

5. mechanical vibrator –

removes air bubbles

• Prepare the impression

1. Disinfection of impression

•________ •__________ •_______________

2. Inspect impression for standing moisture

- surface should be dry of moisture

- gently air dry with A/W syringe

- pooled water will result in voids in cast/model

Manipulation of Gypsum Products

Agents used: ____________, _______________, _______________

Manipulation of Gypsum Products

Measure powder

& water

Add powder to water(over 20 to 30 seconds)

Spatulation

(1 minute at 2 revolutions

per second)

Vibrate (approx 30 sec)

Pour impression(sets in 45 – 60 minutes)

Working time = ~5 min

- mixing & pouring

Setting time:

Initial set = 7 – 10 min

– “loss of gloss” or flow

Final set = 45 – 60 min

– after exothermic reaction (feels warm)

-- wet strength (feels wet & cool to touch)

24 hours = dry strength

Disinfect cast(if nec.)

V. Construction of cast/model - chapter 28

1 2

3 4

5 Or rubber base formers

VI. Separation of cast/model from impression

– chpt 28• Timing –

– Working time mixing & pouring = 5 mins

– Initial set = 7 - 10 min

– Final set - Wet strength = 45 min – 1 hr – after exothermic

reaction – feels cool to touch

– Dry strength = 24 hours** WHY remove the impression immediately after exo reaction???

- Syneresis – dehydrated imp mat’l will shrink & harden making

it very difficult to separate from model/cast.

VIII. Model Trimming – creating symmetrical formAlginate in interproximal undercuts

Blebs or nibs of gypsum on occ surface

Model trimmer

Test #3 Review

Impression Materials & Gypsum

Chapters 8 & 9* Give EDPuzzle a try *

1. Use

2. Desirable qualities & special characteristics

3. Definitions & terminology

4. Flow charts, Common Difficulties handout,

Impression grid

5. Factors effecting the final end-product